1. Field of the Invention
The present invention relates to a duplex printing apparatus, and more particularly, to a paper feed device for reversing a paper feed path for duplex printing.
2. Description of the Related Art
An existing type of duplex printing apparatus includes a main body and a duplex unit installed at one side of the main body. A first delivery roller and a first feed roller are respectively installed at the upper and lower sides of the duplex unit. A driving motor connected to drive the first delivery roller is installed at the inner side of the duplex unit. A solenoid is installed under the driving motor.
When the duplex unit is attached to the main body, the first delivery roller and the first feed roller respectively touch a second delivery roller and a second feed roller installed at corresponding portions of the main body, thereby forming one paper feed path. A central rod of the solenoid moves outward to push an upper portion of a reverse paper guide during simplex printing, and moves inward to release the reverse paper guide during duplex printing.
For simplex printing, a sheet picked up by the revolution of a pickup roller from a paper cassette is fed to a developing unit through a paper guide. An image is formed on one (front) side of the sheet while the sheet passes between a photoconductive drum of the developing unit and a fixing roller. The image is fixed on the sheet when the sheet passes between a heating roller and a pressure roller. Thereafter, the sheet passes through the upper side of a reverse paper guide. The sheet fed along a paper feed path is discharged to the exterior of the main body by the revolution of first and second exit rollers.
For duplex printing, the central rod of the solenoid is retracted. The sheet, printed on the front side, is fed between the delivery rollers through the lower side of the reverse paper guide. The sheet is sensed by a sensor and the delivery rollers are driven in reverse by the driving motor. Then the sheet is fed between the first and second feed rollers along a reverse paper feed path. The sheet is fed between third and fourth feed rollers and is again picked up by the pickup roller. Thereafter, the image forming process for the back side of the sheet is performed.
The duplex printing unit required for duplex printing has a driving motor for driving the delivery rollers and a solenoid for controlling the reverse paper guide. Since the duplex unit includes a driving motor and a number of rollers, its cost is high and its internal construction becomes complicated. Therefore, the duplex unit is not included with the main body of the duplex printing apparatus as standard equipment, but rather is installed as an accessory at one side of the main body as needed by the user.
Other examples of apparatus for duplex printing can be seen in the following U.S. patents. U.S. Pat. No. 4,835,567 to Ogata entitled Paper Re-Feeding Apparatus Of Image Forming Apparatus discusses duplex copying involving guiding the paper through a gate downward by a turn unit onto a stacker with other sheets, where the paper is picked up, sent through switchback rollers and guided by a gate to the developing and fixing units where the paper is printed on the other side. This apparatus has two motors, and requires the activation of several gates as well as stopping and reversal of two pairs of rollers to achieve duplex printing.
U.S. Pat. No. 4,928,127, to Stemmle, entitled Sheet Circulation in a Duplex Printer, discusses a duplex printing apparatus involving the use of reversible rollers. In this design, however, the reversing rollers also serve as discharge rollers, requiring that the duplex printed sheet be partially ejected on the discharge tray during duplex printing. This can cause problems due to interaction with sheet already lying in the discharge tray.
U.S. Pat. No. 4,928,150, to Hatta, entitled Copy Apparatus Having Plural Copy Sheet Discharge Trays for Different Sized Copy Sheets, describes a duplex copying machine which uses reversing rollers partially ejecting the sheet onto a discharge tray to redirect the sheet for the second-side printing. Again, there are potential problems with interactions with sheets on the discharge tray. A solenoid is required to activate the reversing rollers, and the mechanism of this activation is not disclosed.
U.S. Pat. No. 5,055,885, to Yoshikado, entitled Picture Image Forming Equipment, describes a duplex device and in particular the gate where the image-forming channel, paper-discharging channel and paper-reversing channel meet. This apparatus, however, requires a pair of reversing rollers and a lever as part of the gate mechanism, possibly adding to the complexity of the apparatus.
U.S. Pat. No. 5,081,508, to Kotani et al., entitled Paper-Jam Detecting Device, describes a duplex printing apparatus in which the reversing rollers discharge the paper onto the discharge tray. The patent deals with a device for detecting the paper jam caused when the paper is partially ejected and then reversed back for duplex printing, but in the process adheres to papers already in the tray. This device is thus an added layer of complexity to solve a problem created by a certain duplex printing arrangement.
U.S. Pat. No. 5,448,348, to Azeta, entitled Image Forming Apparatus Having A Sheet Material Transport Device, describes a duplex printing apparatus which during duplex printing partially discharges the paper onto the discharge tray before reversing. Again, this device suffers from the problems associated with partial discharge of the paper onto the discharge tray. Additionally, the reversal step requires reversing the motor and changing the position of a flapper.
Based on my observation of the art, I have decided that what is needed is a duplex printing apparatus where the duplex printing mechanism is of simple design and low cost, so that duplex printing can be incorporated cheaply as standard equipment. In particular, it is desirable to avoid reversing the main motor or to have an extra motor to achieve duplex printing.